A Clerk in a Swiss patent office realized that
Newton's law of gravity while right most of the time, was incorrect
when talking about very large and very distant objects. This
man's name was Albert Einstein. Einstein revised Newton's laws
of gravity to make them more accurate. We call what he
developed the theory of relativity.

It is actually two theories. The first is
called Special Relativity. This theory states that it is
impossible to determine whether or not you are moving unless you can
look at another object.

Think about that. If you were in the middle
of outer space far from any other objects, how would you know whether
or not you were moving? All movement is relative to other
objects. For example, right now relative to your computer you
are not moving at all, but relative to the distant quasars you are
moving at nearly the speed of light. Relative to the Earth, most
meteorites move at about 25,000 miles an hour (40,233 km), but if you were
standing on a meteorite looking at another meteorite going in the
same direction as you and at the same speed, it would not appear to
move at all.

Special Relativity also says that the speed of
light is always constant. This means that no matter what you do
to light it will always go the same speed. (Scientists are
learning how to make light go faster, but it is very difficult).

The Theory of General Relativity is the one which
redefined the laws of gravity. It says that it is impossible to
tell the difference between gravity and the force of inertia from a
moving object.

In other words, if you climb inside of a spinning
spacecraft, the inertia will cause you to move towards the outside
walls in a way that would feel just like gravity. This is why
future spacecraft designs often have large spinning cylinders
attached to them.

The Theory of General Relativity also says that
large objects cause outer space to bend in the same way a marble laid
onto a large thin sheet of rubber would cause the rubber to
bend. The larger the object, the further space bends.
Just like a bowling ball would make the rubber sheet bend much
more than the marble would.